1. Technical Fields
The present invention relates to a flame retardant, high precision resin mechanical part for use in an office automation machine, which is made by injection molding a thermoplastic resin composition. More particularly, the present invention is concerned with a flame retardant, high precision resin mechanical part for use in office automation machines required to function with high accuracy and high precision, which part is made by injection molding a thermoplastic resin composition comprising: (A) an amorphous thermoplastic resin; (B) an inorganic filler in a scale form; and (C) a specific phosphoric acid ester. The high precision resin mechanical part of the present invention is free from conventionally experienced disadvantages, such as volatilization of a flame retardant contained in the resin, smoking caused by the volatilization of the flame retardant and occurrence of MD (mold deposit) during the production thereof by molding, and bleeding of the flame retardant during the use thereof. Further, the mechanical part of the present invention is unlikely to suffer from warpage, and has an advantageously small shrinkage ratio during the production thereof by molding. Therefore, the mechanical part of the present invention has not only high precision but also various excellent mechanical properties, such as extremely low anisotropy in linear expansion coefficient, and excellent vibration characteristics. The high precision resin mechanical part of the present invention can be advantageously used as a mechanical part for high precision machines which are required to function with high accuracy and high precision even under various stringent conditions. Examples of such high precision machines include computers, game machines, sound-reproducing systems, audio-visual machines, copiers, printers, facsimile machines, personal computers, word processors, portable communication apparatus and composite machines composed thereof.
2. Technical Background
Recently, in various fields, such as automobiles, office machines, computers and household electric appliances, it has been attempted to substitute a resin part for parts which have conventionally been made from a metallic material, such as sheet metals and die-casted aluminum. Such an attempt has been made because a part made from a resin is advantageous in that such a resin part has not only light weight, but also can be produced with high productivity at low cost. Therefore, there is an increasing demand for reinforced resins which can be substituted for metallic materials in producing a mechanical part. Particularly, there is a strong demand for reinforced resins which can be used for producing mechanical parts for OA (office automation) machines, such as copiers, printers, facsimile machines, CD-ROMs, personal computers, word processors and communication apparatuses. The copiers, printers and facsimile machines are OA machines which are equipped with a printing mechanism, such as a dry-type, diazo-type, LB (laser beam)-type, BJ (bubble jet)-type, dot-type or heat sensitive-type printing mechanism.! In accordance with such a strong demand for reinforced resins, it has been intensively studied to develop amorphous thermoplastic resins (e.g., a reinforced, flame retardant polyphenylene ether resin and a reinforced, flame retardant polycarbonate resin) which have excellent mechanical properties and excellent molded form characteristics, so as to produce from such resins mechanical parts which have conventionally been produced from metallic materials, such as sheet metals and die-casted aluminum.
It is a requisite that a resin mechanical part of an OA machine have excellent molded form characteristics, mechanical properties (e.g., rigidity and strength), heat resistance, flame retardancy, dimensional precision and dimensional stability.
Among these properties, the requirements for dimensional precision and dimensional stability are most strict. For example, with respect to the mechanical part of a CD-ROM, when a traverse base, which guides and holds an optical lens unit, has a dimensional strain, the misreading of data from the CD occurs. With respect to the internal mechanical parts of OA machines having a laser beam-type printing mechanism, such as a copier, a printer and a facsimile machine, when such mechanical parts have a dimensional strain, the misreading of data is caused, thereby leading to disadvantages, such as blurring of the printed image. Further, with respect to the internal mechanical parts of OA machines having a diazo-type, bubble jet-type or heat sensitive-type printing mechanism, when such mechanical parts have a dimensional strain, the printed letter becomes blurred.
With respect to such mechanical parts as mentioned above, it is required that the dimensional precision thereof be such that any errors or deviations must be only on the order of several tens of .mu.m.
When it is attempted to produce a mechanical part of an OA machine from an amorphous thermoplastic resin in accordance with a conventional method, the following problems are encountered.
As a conventional technique for imparting a resin with high rigidity and strength, there has been known a method in which a fibrous inorganic filler, such as a glass fiber, is blended with a resin. This technique has disadvantages as follows. That is, when a fibrous inorganic filler having a large aspect ratio (length/thickness), such as an inorganic fiber or a whisker, is added to the resin, and the resultant resin composition containing such a fibrous inorganic filler is subjected to injection molding, the fibrous inorganic filler is oriented along the direction of flow of the resin. Accordingly, an obtained molded article inevitably becomes anisotropic with respect to the rigidity, strength, shrinkage ratio (occurring during molding thereof) and linear expansion coefficient. Therefore, it is likely that the rigidity and strength of the molded article become poor and that the shaped article suffers warpage which leads to a dimensional deformation or dimensional strain of the molded article. Further, this technique is also disadvantageous in that the anisotropy of linear expansion coefficient causes the molded article to be susceptible to a change in temperature. Specifically, the molded article, which is anisotropic in linear expansion coefficient, easily suffers dimensional deformation when the temperature changes, so that it is poor in dimensional stability.
On the other hand, when a conventional inorganic filler having a small aspect ratio, such as glass beads and calcium carbonate, is used, the obtained molded article is improved with respect to dimensional precision and dimensional stability. However, this molded article also is disadvantageous in that it has poor rigidity and strength.
For the purpose of improving the flame retardancy of an amorphous thermoplastic resin, a flame retardant is generally used. For example, a conventional halogenated aromatic compound, such as tetrabromobisphenol A (TBA) and polybromobiphenyl oxide (PBBO), has been widely used to improve the flame retardancy of an amorphous thermoplastic resin with the exception of polyphenylene ether resin. However, with respect to not only the halogen type flame retardant, but also antimony trioxide (Sb.sub.2 O.sub.3), which is generally used as an auxiliary flame retardant for the halogen type flame retardant, is undesirable in view of the adverse influence on the environment and the safety to human body. Therefore, in the market, there is an increasing tendency to refrain the use of the above-mentioned halogen type flame retardant and antimony trioxide. Meanwhile, there is known a polyphenylene ether resin composition or a polycarbonate resin composition, in which, as a non-halogen type flame retardant, a phosphorus compound is employed (see, for example, G.B. Patent Application Publication No. 2043083 and U.S. Pat. No. 5,204,394). Specifically, an organic phosphoric acid ester compound, such as triphenyl phosphate, cresyl diphenyl phosphate and tricresyl diphenyl phosphate, has been widely used as the flame retardant. However, the resin composition containing the organic phosphoric acid ester compound is disadvantageous in that the phosphoric acid ester compound is likely to be volatilized or generate smoke during molding of the resin composition containing the same and that MD (mold deposit) is likely to occur on the inner surface of a mold cavity. Further, a molded article produced from the resin composition containing such a phosphoric acid ester compound has a defect in that the phosphoric acid ester compound bleeds out on the surface of the molded article (bleeding), thereby causing the molded article to suffer discoloration, blistering, cracking and the like.
As a method for solving the above-mentioned problems, it has been proposed to employ, as a flame retardant, an organic phosphorus compound having a high molecular weight. Specifically, it has been attempted to employ, as a flame retardant, a phosphorus compound having a high molecular weight, such as tri(2,6-dimethylphenyl)phosphate, resorcinol-bisdiphenyl phosphate and tribiphenyl phosphate (see, for example, International Patent Application Publication No. 94/03535 corresponding to European Patent Application Publication No. 0 611 798). However, these organic phosphorus compounds are disadvantageous in that such a compound must be used in a large amount in order to render flame retardant the resin, and that when the resin composition, which is rendered flame retardant with a phosphoric acid ester compound, is subjected to molding, corrosion of a metallic mold is caused during the molding thereof. Further, a mechanical part produced from such a resin composition has poor resistance to moisture and heat. Specifically, when the above-mentioned mechanical part is placed under high temperature and high humidity conditions, the resin of the mechanical part absorbs water and/or the flame retardant contained in the resin undergoes denaturation, thereby deteriorating various properties of the resin, such as electrical characteristics, flame retardancy and dimensional stability. As is understood from the above, the conventional techniques have various problems and it has been impossible to substitute a resin part containing a non-halogen type retardant for a metallic mechanical part for an OA machine. That is, it has been extremely difficult to produce a mechanical part, from a resin composition containing a non-halogen type flame retardant, which part has excellent mechanical properties, dimensional precision, molded form characteristics and flame retardancy.